The purposes of the formation of a chromate conversation coating on the surface of aluminum are to provide corrosion resistance, improve adhesion of coatings and for aesthetic reasons. A conversation coating improves the adhesion of coating layers such as paints, inks, lacquers and plastic coatings. A chromate conversion coating is typically provided by contacting aluminum with an aqueous composition containing hexavalent or trivalent chromium ions, phosphate ions and fluoride ions. Growing concerns have arisen regarding the pollution effects of the chromate and phosphate discharged into rivers and waterways by such processes. Because of high solubility and the strongly oxidizing character of hexavalent chromium ions, conventional chromate conversion processes require extensive waste treatment procedures to control their discharge.
In addition to concerns with respect to waste products, when typical prior art chromium based treated materials are stored subsequent to treatment, prior to painting, it is well known that in some cases there is a deterioration in adhesion performance.
Attempts have been made to produce an acceptable chromate-free conversion coating for aluminum. For example, U.S. Pat. No. 4,313,769, which issued to Frelin et al., discloses an acidic aqueous coating solution containing relatively low concentrations of zirconium, hafnium, or titanium and fluoride which includes a surfactant to improve resistance of the treated aluminum to hot water discoloration. Similarly, U.S. Pat. No. 4,370,177, which issued to Frelin et al., discloses an aqueous coating solution containing relatively low concentration of zirconium, hafnium or titanium and a fluoride which includes at least two surfactants.
U.S. Pat. No. 3,912,548, which issued to Faigen, discloses a composition which produces a corrosion resistant siccative finish bonding surface which comprises interalia a polyacrylic acid and a soluble zirconium compound such as alkaline metal, ammonium fluozirconate or ammonium zirconium carbonate applied at a pH of from 6 to 8. U.S. Pat. No. 4,191,596, which issued to Dollman et al., discloses a composition for coating aluminum which comprises a polyacrylic acid and H.sub.2 ZrF.sub.6, H.sub.2 TiF.sub.6 or H.sub.2 SiF.sub.6. The claims of the '596 patent are directed to a mixture of from about 0.5 to about 10 grams per liter of the polyacrylic acid and from about 0.2 to about 8 grams per liter of the metal acid applied at a pH of less than about 3.5. Examples of the '596 patent are limited to the use of from 2.05 to 4.11 grams per liter of the polyacrylic acid, and from 0.85 to 1.942 grams per liter of the metallic acids. Thus the ratios of polyacrylic acid: metal acid disclosed by Dollman range of from 2.1:1 to 4.8:1. No details with respect to the treatment time or coating weight are given for examples 1-3, while examples 5-7 were treated for 30 seconds. The '596 patent also discloses the use of HF in combination with polyacrylic acid and H.sub.2 TiF.sub.6 in a concentrated replenishing solution.